As recording media which are used in a computer backup unit and the like, a so-called coating type magnetic recording medium in which a magnetic coating material prepared by dispersing and kneading a magnetic powder, a binder and various additives in an organic solvent is coated on a non-magnetic support and dried is the main current because of its excellent productivity and generalization.
In recent years, realization of small size and large capacity is advancing, and following this, high-density recording is being eagerly demanded in the foregoing coating type recording medium, too.
In recent years, in magnetic recording and reproducing systems for recording and reproducing a computer data, a system integrated with a thin film magnetic head is put into practical use. Since the thin film magnetic head is readily miniaturized or processed into a multitrack head, a multitrack stationary head of a thin film magnetic head is frequently utilized especially in a system using a magnetic tape as a recording medium. By utilizing a thin film magnetic head, it becomes possible to enhance the track density or enhance the recording efficiency due to the miniaturization and to realize high-density recording. Also, it becomes possible to enhance a data transfer speed due to the multitracking. The thin film magnetic head is divided roughly into an induction type head which is responsible to a time change of magnetic flux and a magnetoresistive head (MR head) utilizing a magnetoresistive effect which is responsible to a size of magnetic flux. Since the induction type head is of a planar structure, the number of turns of head coil is small, and it is difficult to increase a magnetomotive force, leading to a problem that a sufficient reproducing output is not obtained. For that reason, an MR head from which a high reproducing output is readily obtained is used for the reproduction, whereas an induction type head is used for the recording. Such a magnetic head is usually integrated into a system. In such a magnetic recording system, linear recording mode capable of realizing a faster data transfer speed is employed.
For the purpose of realizing high-density recording of a coating type magnetic recording medium, there have been studied and proposed various methods from the viewpoints of use of iron or an alloy magnetic powder composed mainly of iron in place of conventionally used magnetic iron oxide powders, enhancement of magnetic characteristics of a magnetic layer by improvement of a magnetic body such as finely dividing of a magnetic powder or by improvement of filling properties or orientation properties thereof, enhancement of dispersibility of a ferromagnetic powder, enhancement of surface properties of a magnetic layer, and the like.
For example, a method of using a ferromagnetic metal powder or a hexagonal ferrite powder as a ferromagnetic powder for the purpose of enhancing magnetic characteristics is disclosed in, for example, JP-A-58-122623, JP-A-61-74137, JP-B-62-49656, JP-B-60-50323, U.S. Pat. Nos. 4,629,653, 4,666,770 and 4,543,198.
JP-B-1-18961 discloses a ferromagnetic powder having a specific surface area of from 30 to 55 m2/g, a coercive force of 1,300 Oe or more, and an amount of saturation magnetization of 120 emu/g or more as a metallic magnetic powder having a long axis size of from 0.05 to 0.2 μm and an axial ratio of from 4 to 8 and provides a fine metal powder having a small specific surface area. Also, JP-A-60-11300 and JP-A-60-21307 disclose a manufacturing process of a fine iron α-oxyhydroxide acicular crystal which is suitable for a ferromagnetic powder, especially a ferromagnetic metal powder, and the latter discloses that a ferromagnetic metal powder having an Hc of from 1,450 to 1,600 Oe and a σs of from 142 to 155 emu/g can be produced from a goethite having a long axis length of from 0.12 to 0.25 μm and an axial ratio of from 6 to 8. JP-A-9-91684 proposes to use ferromagnetic metal particles containing ferromagnetic metal particles having an average long axis size of from 0.05 μm to 0.12 μm and an axial ratio of 8 or more in a proportion of not more than 5.0% of the whole of the ferromagnetic metal particles, or ferromagnetic metal particles containing ferromagnetic metal particles having an axial ratio of a crystallite constituting the particles of 4 or more in a proportion of not more than 17.0% of the whole of the ferromagnetic metal particles. However, if particles having a small axial ratio are intermixed, a ferromagnetic powder having a high Hc is hardly obtained, and the S/N and the overwrite characteristics are insufficient.
Further, JP-A-6-340426 and JP-A-7-109122 disclose mono-dispersed spindle type hematite particles using a hematite nuclear crystal, iron hydroxide and specific ions and an extremely fine ferromagnetic powder obtained by reducing the foregoing hematite particles.
Also, for the sake of enhancing the dispersibility of a ferromagnetic powder, it is proposed to use various surfactants (as disclosed in, for example, JP-A-52-156606, JP-A-53-15803 and JP-A-53-116114) or to use various reactive coupling agents (as disclosed in, for example, JP-A-49-59608, JP-A-56-58135 and JP-B-62-28489).
Also, JP-A-1-239819 discloses a magnetic powder prepared by adhering a boron compound, an aluminum compound or an aluminum compound and a silicon compound successively onto the particle surface of magnetic iron oxide and describes that the magnetic characteristics and dispersibility are improved. Further, JP-A-7-22224 discloses a magnetic metal powder containing not more than 0.05% by weight of an element belonging to the group 1a of the periodic table and optionally containing from 0.1 to 30 atomic % of aluminum based on the total amount of metal elements and further from 0.1 to 10 atomic % of a rare earth element based on the total amount of metal elements, wherein the residual amount of an element belonging to the group 2a of the periodic table is not more than 0.1% by weight and describes that a high-density magnetic recording medium having good storage stability and magnetic characteristics is obtained.
Further, for the sake of improving the surface properties of a magnetic layer, there is proposed a method of improving a surface forming processing method of a magnetic layer after coating and drying (as disclosed in, for example, JP-B-60-44725).
On the other hand, for the purpose of achieving a high recording density of a magnetic recording medium, realization of shortening a wavelength of signals to be used is eagerly being advanced. However, if the length of a region where signals are recorded becomes a size comparable to the size of a magnetic body as used, it is impossible to prepare a distinct magnetization transition state, and therefore, it is substantially impossible to execute recording. For that reason, it is required to develop a magnetic body having a sufficiently small particle size against the shortest wavelength to be used, and it has been pointed for many years to finely divide the magnetic body.
In a metal powder for magnetic recording, the particle shape is made acicular to impart shape anisotropy, thereby obtaining a targeted coercive force. For the sake of achieving high-density recording, it is well known by those skilled in the art that it is necessary to make the surface roughness of a medium obtained by finely dividing a ferromagnetic metal powder small. However, if the metal powder for magnetic recording is finely divided, following this, the axial ratio is lowered, whereby a desired coercive force is not obtained.
In a magnetic recording system integrated with an MR head, there are the following problems with respect to the adaptability of the MR head with a magnetic recording medium to be used in this system. That is, in the case where a magnetic recording medium having a relatively thick (0.3 μm) magnetic layer is used, since the magnetic flux of the magnetic layer becomes high, a reproducing output is excessively high so that the MR head is saturated, whereby the reproducing waveform is warped. As a result, a sufficiently high S/N value is not obtained, and an error rate likely increases. Also, in general, for the sake of achieving a high recording density, it is desired that the recording and reproducing waveform (isolated reproducing inverse waveform) is sharper (the half value width of the waveform is small). However, it has been noted that in a magnetic recording medium having a relatively thick magnetic layer, the half value width of the recording and reproducing waveform becomes large, whereby a sufficient high recording density is not obtained. On the other hand, in the case where a magnetic recording medium having a very thin (0.03 μm) magnetic layer is used, the recording and reproducing waveform is warped. As a result, it has been noted that a high S/N value is not obtained, too and that the reproducing output itself is likely lowered. Also, in a magnetic recording medium which is suitably used in a magnetic recording and reproducing system integrated with an MR magnetic head capable of achieving recording at a fast data transfer rate and at a high density, it is considered that an extremely fine ferromagnetic powder is necessary. However, if the particle becomes fine, it has become difficult to manufacture a ferromagnetic powder which is satisfied with both magnetoelectric transform characteristics and storage stability (weather resistance). With respect to a magnetic powder to be used in a high-density recording region, the magnetic powder must be a fine particle having excellent magnetic characteristics. However, in a magnetic powder, if the size is small, a portion of fine particles exhibiting superparamagnetic properties becomes high, whereby the magnetic characteristics are markedly lowered.
In the light of the above, with respect to magnetic recording media using a ferromagnetic metal powder, which have hitherto been employed, various studies are made. However, it is the actual situation that ones from which good magnetoelectric transform characteristics are obtained and which are satisfactory as an excellent high-density recording medium have not been obtained yet.